The present invention relates to a nitrogen adsorbent which selectively adsorbs nitrogen from a nitrogen-containing gas such as air, and a method of using the same.
A pressure swing adsorption method (PSA method) is one of methods for selective adsorption and separation of nitrogen from a nitrogen-containing gas. This PSA method uses a nitrogen adsorbent such as a zeolite, to obtain oxygen by adsorbing and separating nitrogen from air, for example. The PSA method is conducted by repeating a cyclic pressure change, in other words selectively adsorbing nitrogen by use of a nitrogen adsorbent under a high pressure and then returning to a low pressure to thereby release nitrogen from the nitrogen adsorbent and to regenerate the nitrogen adsorbent.
The zeolite used in the PSA method has cations associated in a crystal thereof and the cations exert an electrostatic attraction on a gas molecule (nitrogen) to be adsorbed. Thus, the zeolite has a property to adsorb more highly polar molecules. By using these properties and changing the cations associated therewith, zeolites with a good adsorption performance have been found. U.S. Pat. No. 3,140,932, for example, discloses an X-zeolite having one cation of Ba, Sr and Ni and exhibiting excellent nitrogen adsorption. U.S. Pat. No. 3,140,933 also discloses a technique concerning nitrogen adsorption which uses a zeolite having an apparent pore size of not less than 4 xc3x85 and containing Li+ as a cation. Further, TOKKOHEI (Japanese published examined patent application) 5-25527 discloses a technique concerning nitrogen adsorption which uses a zeolite having an SiO2/Al2O3 ratio of from 2.0 to 2.5 and not less than 88% of Li+ cations associated. In TOKKOHEI 7-57300, a zeolite is disclosed which has an improved adsorption performance by mixing divalent cations in addition to a Li cation.
However, the aforementioned conventional zeolites require an extremely high association ratio of cations such as Li+ in order to ensure a high adsorption performance.
Therefore, the zeolites require many cations such as Li+ for ion-exchange, causing a problem of a high production cost. Further, an X-zeolite has an extremely high affinity with water and once it adsorbs, even a very small amount of adsorbed water remarkably deteriorates its performance, so that activation by dehydration is required. Such activation generally needs a high temperature of not less than 400xc2x0 C. Thus, zeolites inferior in heat resistance such as an Al-rich zeolite and a zeolite having an Li+ cation of a high charge density, are difficult to handle in activation, and in the worst case their performances are deteriorated.
In view of the foregoing, it is an object of the present invention to provide a nitrogen adsorbent exhibiting an improved heat-resistance and an improved nitrogen separation factor with a less production cost and a method of using it.
In accordance with an aspect of the present invention in achieving the aforementioned object, provided is a nitrogen adsorbent comprising a crystalline X-zeolite having a faujasite structure with an SiO2/Al2O3 ratio of less than 3.0, wherein the crystal contains at least one trivalent element of the group consisting of Fe, B and Ga and has (AlO4)5xe2x88x92 tetrahedral units thereof associated with cations.
Further, in accordance with another aspect of the present invention, provided is a method of using a nitrogen adsorbent wherein the nitrogen adsorbent as described above adsorbs nitrogen after being heated under a vacuum.
The nitrogen-selective adsorption performance of a zeolite has the property of adsorbing more highly polar molecules since the cations associated in a zeolite crystal, as mentioned above, exert an electrostatic attraction on a gas molecule (nitrogen) to be adsorbed. Particularly, the X-zeolite is one kind of zeolite having large pores and the smallest SiO2/AlO2O3 ratio. Thus, the X-zeolite has an appropriate space to adsorb gas molecules and a large number of cation sites.
The inventors of the present invention have found that the X-zeolite containing at least one trivalent element selected from the group consisting of Fe, B and Ga, and (AlO4)5xe2x88x92 tetrahedral units associated with cations in the crystal thereof, has a larger number of cation sites contributing to adsorption and a high nitrogen adsorption performance, such as nitrogen-oxygen separation performance. In addition, the zeolite containing at least one trivalent element selected from the group consisting of Fe, B and Ga may have improved heat-resistance. Particularly, the zeolite containing Fe as the trivalent element adsorbs, in terms of adsorption amount of nitrogen and oxygen, much less oxygen than nitrogen. With such a property, during nitrogen-oxygen separation in a PSA method, it is found that an excellent separation performance is exhibited and the oxygen generation ratio becomes extremely high. Further, although some materials have an excellent performance only in nitrogen adsorption, the nitrogen adsorbent of the present invention has an excellent separation factor, and so far there is no other material exhibiting such an excellent performance. Thus, with the zeolite having a high separation factor, the efficiency unit in a PSA method (amount of electricity per one unit of oxygen generation volume) can remarkably be improved and thus oxygen can be generated by less energy in comparison with a conventional one.
In accordance with the present invention, when Li is used as a cation, an excellent nitrogen adsorption performance is exhibited. In other words, among the cations contributing to adsorption, Li+ has an ion radius of 0.60 xc3x85, which is the smallest among alkali metals. Therefore, Li+ has the highest charge density among monovalent cations and a strong interaction with a polar substance to thereby attract cations and to create an electrostatic field by its bonding balance. Such an electrostatic field attracts and selectively adsorbs molecules with quadrupole moment such as nitrogen, resulting in the zeolite exhibiting an excellent nitrogen adsorption performance.
Moreover, the inventors have found that when the nitrogen adsorbent of the present invention has not less than 60% to less than 88% of its (AlO4)5xe2x88x92 tetrahedral units associated with Li+ cations, an excellent adsorption performance is exhibited. Accordingly, the nitrogen adsorbent of the present invention can obtain the excellent adsorption performance even at a relatively lower cation association ratio, thereby decreasing production costs.
Furthermore, the inventors have found that adsorption performance is improved by heating the adsorbent as described above under a vacuum and then adsorbing nitrogen. It has been found that a relatively high temperature, which is not less than 400xc2x0 C. to not greater than 600xc2x0 C., is suitable for heating under a vacuum.
Embodiments of the present invention will next be described in detail.
The nitrogen adsorbent of the present invention selectively adsorbs and thereby separates nitrogen from a nitrogen-containing gas. The nitrogen-containing gas intended is typically air. In this case, nitrogen is adsorbed and separated from air for generating oxygen. The adsorbent also may be used for separating nitrogen from gas mixtures of nitrogen with oxygen, argon, helium, neon, hydrogen and the like as well as from air.
The nitrogen adsorbent of the present invention comprises a crystalline X zeolite. The X zeolite has a faujasite structure which has an SiO2/Al2O3 ratio of less than 3.0. The upper limit for the SiO2/Al2O3 ratio is preferably not greater than 2.5, more preferably 2.0.
The nitrogen adsorbent has at least one trivalent element of the group consisting of Fe, B and Ga in the crystal thereof. It is considered that, due to the presence of these trivalent elements in the crystal, the adsorbent exhibits a high separation performance in obtaining oxygen by adsorbing and separating nitrogen from air, and exhibits enhanced heat resistance. It is also considered that these trivalent elements in the crystal, for reason described hereinafter, may not be present in the form of an oxide or a cation, but may be present in a framework of the crystal. In other words, it is thought that a part of the (AlO4)5xe2x88x92 tetrahedron of the X zeolite is replaced by at least one trivalent element selected from the group consisting of Fe, B and Ga, the number of cation sites contributing to adsorption is maintained, and therefore the adsorbent shows a high separation performance. In addition, because the (AlO4)5xe2x88x92 tetrahedron is replaced by the trivalent elements, the Al component is decreased, which thereby enhances heat resistance.
Among the above mentioned trivalent elements, Fe is particularly suitable. When the crystal has Fe therein, an amount of adsorbed oxygen becomes much smaller than an amount of adsorbed nitrogen during the adsorption and separation of nitrogen from air. Thus, the separation factor is excellent in obtaining oxygen by a PSA method, resulting in a considerably high oxygen generation ratio. The separation factor (N2/O2) is represented by the following formula (1).
(N2/O2)=(NN2/YN2)/(N2O/Y2O)xe2x80x83xe2x80x83(1)
NN2: amount of adsorbed N2 under a partial pressure (608 Torr) of nitrogen in air
YN2: molar fraction of nitrogen in air (0.8)
N2O: amount of adsorbed O2 under a partial pressure (152 Torr) of oxygen in air
Y2O: molar fraction of oxygen in air (0.2)
The nitrogen adsorbent of the present invention has a cation associated with the (AlO4)5xe2x88x92 tetrahedral unit thereof. The cation is in a position near (AlO4)5xe2x88x92 with an excessive negative charge for neutralizing it and associating therewith in the crystal. The cation exerts an electrostatic attraction on a gas molecule to be adsorbed and thereby adsorbs a number of nitrogen molecules more polar than oxygen.
The cation is not particularly limited, examples thereof include as a monovalent cation Li+, Na+, K+, Rb+, and Cs+, as a divalent cation Mg2+, Ca2+, Sr2+, and Ba2+, and as a trivalent cation lanthanoids such as La3+ and Ce3+, and Sc3+, Y3+, B3+, Al3+, and Ga3+. Among these, Li+ is particularly suitable. Li+ has an ion radius of 0.60 xc3x85, the smallest among alkali metals. Therefore, Li+ has a high charge density and a strong interaction with a polar substance to thereby attract cations and to create an electrostatic field by its bonding balance. Such electrostatic field attracts and selectively adsorbs a molecule with a polar moment such as nitrogen. A divalent cation such as Ca2+ has a higher charge density and adsorbs more polar molecules such as nitrogen than Li+, but it also adsorbs more oxygen in comparison with Li+, thereby decreasing a separation factor (N2/O2).
Li+ ion-exchanged X zeolite, as mentioned above, promotes adsorption of nitrogen molecules by a Li+ ion associated with (AlO4)5xe2x88x92 tetrahedral unit, and thus in general with a higher association ratio of the Li+ ion, the zeolite exhibits a good adsorption performance. It is preferred that not less than 88% of (AlO4)5xe2x88x92 tetrahedral unit is associated therewith. However, according to the present invention, the nitrogen adsorbent can exhibit good adsorption performance even when Li+ ions are associated with not less than 60% to less than 88% of the (AlO4)5xe2x88x92 tetrahedral unit. Conventionally, the amount of Li+ ions used is large for a higher association ratio of Li+ ions, but in accordance with the present invention, a relatively low association ratio of Li+ ions still allows good adsorption performance, thereby reducing production costs.
The nitrogen adsorbent of the present invention may, for example, be produced by the following process.
First, sodium silicate and sodium aluminate as starting materials are adjusted with sodium hydroxide, potassium hydroxide and the like to have an SiO2/Al2O3 ratio of less than 3.0 and the formulation mentioned below. To the adjusted solution are added boron oxide, sodium tetraborate and the like as B source, gallium oxide, gallium nitrate and the like as Ga source, and iron nitrate, iron chloride and the like as Fe source. The adjusted solution, after adding a seed crystal thereto, is heated at a temperature of 40 to 100xc2x0 C. for 24 to 120 hours for aging. The resultant solution is next heated and maintained at a temperature of 60 to 100xc2x0 C. for crystallization. Consequently, a nitrogen adsorbent comprising X-zeolite containing Fe is obtained. In the aforementioned condition, the nitrogen adsorbent comprises Na and/or K-type X-zeolite having Na+ and K+ as a cation associated with (AlO4)5xe2x88x92 tetrahedral unit of the X zeolite.
Composition
SiO2/(Al2O3+Fe2O3)=1.6xcx9c3.0
Fe2O3/(Al2O3+Fe2O3)=0 0.3
(Na2O+K2O)/(Al2O3 +Fe2O3)=5.0xcx9c10.0
K2O/(Na2O+K2O)=0xcx9c0.5
H2O/SiO2=40xcx9c80
Further, in the case of obtaining a nitrogen adsorbent comprising Li type X-zeolite with Li+ as the associated cation, the Na and/or K-type X-zeolite is subjected to ion exchange treatment for ion-exchanging Na+ and K+ with Li+ and consequently. Li+ is the associated cation with the (AlO4)5xe2x88x92 tetrahedral unit.
Generally, a nitrogen adsorbent, before its use, is activated at about 400xc2x0 C. to remove adsorbed water molecules to not greater than about 1% by weight. The nitrogen adsorbent of the present invention, before its use, is heated under a reduced pressure at 400 to 600xc2x0 C. for several hours, and therefore the adsorbent exhibits an improved adsorption performance than one which is activated only by removing adsorbed water molecules. The reason for the above has not as yet been made clear. The above property has not been seen in a conventional X-zeolite without Fe and the like, and thus the presence of Fe and the like in the crystal may be a reason.